1. Field of the Invention
The present invention relates to optical fiber modules such as dispersion compensation or amplifier modules, in particular such modules which have reduced multi-path interference.
2. Technical Background
The continued build-out of fiber optic telecommunications infrastructures has spawned a vast array of fiber optic components designed to modify, condition, or route the transmitted signal. Such components include optical fiber couplers, optical fiber amplifiers, and dispersion compensating modules. Typically, these components are in the form of modules. That is, the component is contained in an enclosure, or box, and is inserted into a fiber optic transmission line as a discrete device. Such modules may themselves be composed of one or more smaller modules, or sub-units.
Fiber optic modules often contain a specialty fiber that performs all or a portion of the function of the module. In the case of optical fiber amplifiers and dispersion compensating modules, these specialty fibers are gain fibers and dispersion compensating fibers, respectively. Gain fibers and dispersion compensating fibers may be contained in the largest unit of the module, or the specialty fiber may be contained in a smaller module, or sub-unit. Often the specialty fiber is many meters long. To facilitate placement in the module containing it, the fiber is typically held in a coiled configuration.
To minimize optical loss, the module containing the specialty fiber is typically fusion spliced into the optical path to reduce optical loss at the splice point. That is, the input end and the output end of the specialty fiber are fusion spliced to other fibers. Such other fibers may be transmission fibers, or they may be pigtails connected to other fiber optic devices such as, for example, semiconductor lasers. Less often, mechanical splices may be used. In either case, however, an interface is created between the input or output end of the specialty fiber, and the other fiber or component to which it has been mated.
Optical fiber amplifiers consist of an optical gain fiber with a core containing one or more optical gain dopants such as rare earth ions. The gain fiber may be contained within the amplifier enclosure only, or it may be a part of a smaller module contained within the amplifier enclosure. In operation, the amplifier receives an optical signal of wavelength λs and pump power of wavelength λp which are combined by means such as one or more pump/signal wavelength division multiplexer (WDM) couplers located at one or both ends of the amplifier.
A key characteristic of a fiber optic amplifier is its noise figure, which is the ratio of the signal-to-noise ratio at the amplifier input to the signal-to-noise ratio at the amplifier output. The noise figure characterizes the amount of noise the fiber amplifier adds to the signal λs. The signal λs is input to the amplifier and follows a primary signal path to the amplifier output. The signal light that follows the primary signal path is hereinafter referred to as the primary signal light. Fundamental unavoidable noise is generated in the primary signal light by spontaneous emission produced by the gain fiber. The spontaneous emission noise gives rise to a minimum noise figure. Excess noise is generated in the amplifier when a portion of the primary signal light follows a secondary path and arrives at the output at some time delay relative to the signal light in the primary path, thereby generating one or more secondary signals that may interfere with the primary signal. Such interference is termed multi-path interference (MPI). MPI may form as a result of multiple reflections of the primary signal light within the amplifier, coupling of the primary signal light to higher order modes, scattering of the primary signal light from discrete or distributed sites within the optical fiber, or even four wave mixing. More specifically, MPI results when a delayed secondary signal is re-coupled into the original primary signal. When re-coupling occurs, the total optical intensity can be described by,I∝E12+E22+E12E22 cos([ω1−ω2]t+[φ1(t) −φ2(t)])where E1 and E2 are the field amplitudes of the primary and delayed secondary signal, respectively. The oscillatory term in the above equation, that is the term containing the cosine function, is the root of MPI, and time-dependent differences between the frequencies ω or the phases φ result in receiver noise. Coherent crosstalk occurs when the path delay is within the coherence time of the transmission laser, implying that ω1=ω2. In the case of coherent crosstalk, noise is driven only by phase difference, resulting both from the output characteristics of the transmission lasers as well as the environmentally-dependent phase relationships of the separate optical paths. Alternatively, incoherent crosstalk results when the path delay incurred by the secondary path is longer than the coherence time of the laser, in which case the recombined signal may contain variations due to both frequency and phase fluctuations.
The gain fiber of an optical fiber amplifier has an input end and an output end, and typically the length of gain fiber is formed into a coil to facilitate placement of the fiber within the amplifier, or module enclosure. The input end and the output end of the gain fiber may be connected to other components within the optical amplifier or module, such as, for example, optical coupler pigtails, or one or both ends of the gain fiber may be connected to other fibers. Connection of the gain fiber is usually made by fusion splicing the input and output ends of the gain fiber to the fiber pigtail of another amplifier component or to a transmission fiber. One particular source of MPI occurs when signal light is perturbed by the splice point at the input end of the amplifier gain fiber. The primary signal light that is input into the gain fiber may be perturbed sufficiently that some power is coupled out of the fundamental mode of the primary signal light into higher order modes. By fundamental mode we mean the LP01 mode. By higher order modes we mean modes other than the fundamental mode. Primary signal light that is coupled into higher order modes at the splice point at the input end of the gain fiber, hereinafter referred to as the secondary signal light, may be re-coupled into the fundamental mode of the primary signal light by a perturbation at the splice point at the output end of the amplifier gain fiber. The coupled secondary signal light can interfere with the primary signal light and thereby create noise. If the secondary signal light can be prevented from re-coupling into the fundamental mode of the primary signal light, the source of MPI within the amplifier resulting from the re-coupling of optical power from higher order modes, generally termed modal interference, can be eliminated, and the amplifier noise figure can be significantly reduced.
Modal interference is also of concern in the design and operation of other fiber optic modules, including, for example, dispersion compensating modules. Similar to the case of optical amplifiers, the dispersion compensating fiber module contains at least a dispersion compensating fiber. The dispersion compensating fiber has an input end and an output end, and the length of dispersion compensating fiber is typically wound into a first coil to facilitate placement within the module enclosure. The dispersion compensating fiber may be connected to other components within the module, or the compensating fiber may be connected to one or more transmission fibers. Such connections are typically made by fusion splicing the dispersion compensating fiber to the component pigtail or to the transmission fiber. As in the case of optical amplifiers described above, in a dispersion compensating module primary signal light perturbed at the splice point at the input end of the dispersion compensating fiber can be coupled to higher order modes and propagate within the dispersion compensating fiber as secondary signal light. The secondary signal light propagating in the higher order modes may then be re-coupled into the fundamental mode of the primary signal light at the splice point at the output end of the dispersion compensating fiber resulting in interference noise.
As the design of fiber optic modules containing such specialty fiber as, for example, amplifier gain fiber and dispersion compensating fiber, advances, designers seek increasing flexibility for the design of these fibers. For example, one goal of dispersion compensating fiber design involves utilizing larger negative total dispersion accompanied by dispersion slope properties tailored to particular transmission fibers. Several approaches for increased negative dispersion involve setting the fiber cutoff wavelength of the dispersion compensating fiber at a sufficiently high wavelength that the dispersion compensating fiber operates in a multimoded state, thereby allowing the fiber designer to tailor the dispersion of the fundamental mode of the primary signal light without significantly impacting other fiber attributes. In a similar manner, the ability to operate in a multimoded fashion along the length of an amplifier gain fiber presents the fiber and component designer with increased design options. Unfortunately, the presence of multiple strongly-guided modes can potentially cause large levels of modal interference. It would therefore be beneficial if a means of minimizing the presence of modal interference in fiber optic modules could be found.